If data is to be transferred between semiconductor integrated circuits mounted respectively on a printed-circuit board (hereinafter, to be referred to simply as a board) or on different boards, the data signal is often reduced in amplitude and/or shifted in phase due to the attenuation of the signal caused by the parasitic resistance of the package terminal, the line, the cable, etc. on the board, as well as the reflection of the signal from the input/output terminal. And, it is well known that this results in distortion of signal waveforms. In addition, in recent years, a skin effect has often come to appear in high frequency signals as the data transfer rate is improved more and more. The skin effect means a phenomenon that increases the resistance against high frequency signals, since such signals pass near the surface of the subject transmission line. As a result, the distortion of signal waveforms becomes more significant. This has been a conventional problem in data transfer operations.
In such data transfer operations, various types of data are transferred between semiconductor integrated circuits and between boards. For example, there is such data as “0101010101010101. . . ” that includes many high frequency components and such data as “000000111111000000111111. . . ” that includes many comparatively low frequency components. If data includes many low frequency components, signals that pass a transmission path has less skin effect, so that the amplitude becomes large. On the other hand, if data includes many high frequency components, signals that pass a transmission path comes to be affected by such a skin effect significantly, so that the amplitude becomes small. And, such an amplitude change becomes more significant proportionally to the increase of the transmission line in length.
This is why the output amplitude comes to differ among frequency components included in transmission signals in an input circuit consisting of conventional general differential amplifiers, thereby the amplitude is varied significantly. In addition, when the amplitude of an input signal is small, the time required until a predetermined logic threshold is reached at a logic level change is short, thereby the signal delay is suppressed. On the other hand, when the amplitude of an input signal is large, the signal is delayed more, thereby the signal jitter occurs due to the variation of the frequency components included in the signal.
As a result, the output signal of the input circuit comes to be distorted in waveform as shown in FIG. 24 and the opening of the eye-pattern is narrowed. And, if the opening is excessively narrowed, the input circuit will fail in distinguishing of data, thereby accurate data transfer is disabled. This has been another conventional problem.
One of the methods for solving such conventional problems is proposed (for example, in the non-patent document 1, 2001 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS p78–79) as a technique for reducing the distortion of signals with use of inductors L1 and L2 provided at the drain sides of the differential input transistors Q1 and Q2 in a differential input circuit. In addition, there is also an input circuit proposed (for example, in the non-patent document 2, 2002 Symposium On VLSI Circuits Conference Digest of Technical Papers p64–p67) to solve such conventional problems. The input circuit, as shown in FIG. 26, is provided with variable gain amplifiers AMP1 and AMP2 for amplifying the low frequency components of input signals IN and /IN, as well as variable gain amplifiers AMP3 and AMP4 for amplifying the high frequency components of input signals. Low and high frequency components are amplified separately in those amplifiers, then signals are combined through a resistor. After that, the signals are amplified in the amplifiers AMPS and AMP6, then inputted to a differential comparator CPM so as to be distinguished from each other (refer to the non-patent document 2, for example).
[Non-patent Document 1]
2001 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS p78–79
[Non-patent document 2]
2002 Symposium On VLSI Circuits Conference Digest of Technical Papers p64–p67